ELT's presently are primarily used in the location of downed aircraft. When an aircraft goes down, the impact of the crash activates the ELT. ELT's may also be activated manually. The ELT emits a radio signal beacon over the two emergency frequencies (121.5 mHz-civilian and 243.0 mHz-military). These signals are picked up by orbiting satellites, (3-American and 3-Russian) and relayed to Scott Air Force Base in Illinois. This signal beacon gives Scott a general location for a search. Depending on the orbit of the satellite in relation to the signal, the broader the search location may be. Only the satellite in the closest orbit is used to establish a location. The satellite will complete an orbit approximately each 90 minutes. Each orbit will follow a slightly different track and consequently can give a different general location. These differing locations can be as much as 25-35 miles apart.
As an example of this, an aircraft went down in eastern Wyoming. The first location was given to be south of Ayres Natural Bridge (approximately Lat: 42-43-40 Long: 105-36-40). On the next orbit the location had changed to Glendo Lake (approximately Lat: 42-31-20 Long: 105-01-30). Finally on the third orbit, the location was close, by showing Manville (approximately Lat: 42-46-50 Long: 104-36-50). Luckily, the crash was only a forced controlled landing north of the Manville area. Even the close location was not obtained for more than an hour and a half, valuable time lost which the ground team could have used to start an accurate search. If an injury had been involved or if the weather had been bad, this time loss could have been deadly. Even exposure to inclement weather over a given length of time, or to elements of the night, can be deadly to persons stranded.
Commercial airliners also monitor 121.5 mHz, and upon receiving a signal, can give a general location to search personnel. After the general location is established, the nearest search and rescue team (Civil Air Patrol, Sheriff's office, etc.) is notified. In order to come up with an exact location on the signal beacon, these search teams must do a triangulation on the signal. This is done by use of airborne aircraft. Ground vehicles can be used if the terrain permits. The signal needs to be line of site for accurate readings. If the weather is bad, it makes the process of triangulating the signal very difficult or even near impossible. This process can be very time consuming, time that is critical to the survival of the crash victims.
GPS's are distance-based navigational instruments that use satellites to do the triangulation process to arrive at a near exact location. They use a different set of satellites than the ELT's. There are presently (as of Feb. 1993) 22 satellites in orbit. A total constellation of 24 satellites, for a full GPS system, should be fully operational later this year, as published in "AOPAPILOT", Mar. 1993.
The GPS determines its distance from various satellites at known positions, "plots the circles" mathematically, and arrives at a position. By measuring the difference between actual arrival time of the signal and the "timestamp" from the satellite, a distance can be determined. Because the satellites move in space, that distance determines a position on the surface of a sphere with the satellite at the center. By measuring the distance from a second satellite another sphere is determined. This puts us somewhere on the circle where the two spheres intersect. A third satellite measurement produces another "circle of intersection", narrowing down our position to one of two points, just like on the map, while a fourth measurement from yet another satellite resolves the ambiguity and pinpoints your exact position, as published in "EAA's SPORTS AVIATION" February, 1992.
Most of the GPS's presently in use have the capability to track multiple satellites at one time, anywhere from 5 to 8. With the use of this many satellites, GPS's can give a latitude and longitude location with an accuracy of from 15 to, at worst, 100 meters. The Global Positioning System is operated by the U.S. Department of Defense. All GPS systems are subject to accuracy degradation to 100m 2DMS under the Department of Defense imposed Selective Availability program.
Your location is continuously being updated by the GPS system approximately once per second. This information is presented on a LCD display on the GPS unit. GPS's can display a multitude of important information on the LCD display; namely, latitude, longitude and altitude; range, distance and bearing; offset range and bearing from reference waypoint; parallel track and vertical navigation; estimated time of arrival, estimated time en route; ground speed, ground track and desired track; density altitude; true air speed; winds aloft; satellite status; closest point of approach; cross track error; course deviation indicator; sunrise and sunset times, dual timers; and manual and automatic magnetic variation. The aforementioned information is primarily for aircraft navigation and is displayed to the pilot, upon request, with pushbutton operations.
There has not heretofore been provided a method or apparatus for interfacing the positioning information of the GPS system into the radio signal beacon of the ELT. This positioning information may also be integrated into other emergency signal systems in accordance with this invention.